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Influences of Complex Roughness over an Actual Urban Area on Turbulent Flows as Revealed by Large-Eddy Simulations

To investigate the relationship between roughness characteristics and turbulent flows over urban areas, previous studies have mainly focused on flows over surfaces with cube arrays. However, the difference between actual urban buildings and cube arrays is quite large because the height and distribution of urban buildings are spatially inhomogeneous. The purpose of this study is to investigate the effects of actual urban buildings on turbulent flows by conducting large-eddy simulations. The simulations were carried out for flows over the buildings of Kyoto City under a neutrally stratified condition.We used roughness parameters such as averaged building height Have, standard deviation of building height σH, and plan area index λp (the ratio of a plan area of buildings to a unit area) to evaluate building morphological characteristics quantitatively. We conducted two numerical experiments. One is for flows over the actual urban buildings of Kyoto City, which is regarded as the control run. The second is for flows over the surface in which all the building heights are changed to hall (the average building height in computational domain); this is called as the uniform run. We evaluated roughness parameters and Reynolds stress in areas of 500 m by 500 m. Reynolds stress was calculated from 30-minute data. It was found that the difference in Reynolds stress between the control run and the uniform run increases with λp and σH/Have. This difference indicates that building height variation in the control run influences on the magnitude of Reynolds stress. To investigate the influences of urban buildings on large values of instantaneous momentum fluxes which are evaluated as more than 20 times Reynolds stress, we calculated the contribution of larger instantaneous momentum fluxes. The results indicate that the contribution of the large instantaneous momentum fluxes in the control run increases with λp and σH/Have. It suggests that the increase of Reynolds stress is due to large instantaneous momentum fluxes. In addition, from the quadrant analysis of Reynolds stress, it was found that sweep becomes larger with the increase in λp and σH/Have. These results suggest that the variability of building height over actual urban areas intensifies Reynolds stress owing to sweep events.